The present invention relates generally to locking fixation assemblies. More specifically, the present invention relates to a fixed and variable locking screw that can be used in bone fixation assemblies.
In the art of orthopedic fixation, it has become common for surgeons to utilize fixation plates for the treatment of spinal disorders including spinal anomalies, spinal injuries, disc problems, and bone problems. Indeed, within the past several years, the use of fixation plates for the treatment of spinal disorders or for fusion of vertebrae has grown considerably, and spinal plates have found increased use and desirability in the cervical spine as well.
As adequately described by Dr. Howard S. An and Dr. Jerome M. Cotler in the publication entitled Spinal Instrumentation, the upper cervical spine can be approached either anteriorly or posteriorly, depending upon the spinal disorder to be treated. This text discusses the fact that severe complications associated with procedures involving the upper cervical spine can be catastrophic, including injuries to the brain stem, spinal cord, or vertebral arteries, not to mention damage to the esophagus. These complications for upper cervical spine procedures are in addition to the normal complications associated with exposure and fusion of the cervical spine, implantation of a spinal fixation plate, and general disturbance of the spine.
In procedures involving the cervical spine, additional difficulties are encountered as a direct result of the small space in which the surgeon has to operate. When the upper cervical area is dissected, surgeons often find it difficult to maneuver because dissection is intimately close to vital neural, vascular, and visceral structures. As a result, surgeons have little room to operate, and even less room to manually position bone implant structures.
In the typical orthopedic fixation assembly, an orthopedic plate is attached to an adjacent bone by one or more fasteners, typically screws. As is well-known, any device implanted into the human body can cause some type of tissue reaction to the implanted foreign material. Also, in some instances, there are definite anatomic limits to the size of fastener which may be employed for a particular condition. One area where the anatomic limit is particularly notable is in the upper cervical region. For these reasons it is sometimes desirable to use the smallest feasible fasteners for fixating bone segments.
Successful use of spinal instrumentation in the anterior cervical spine is particularly difficult since limited space is available due to anatomic constraints. Any instrumentation implanted in the region must be minimally intrusive, yet have adequate strength to withstand the biomechanical loads to which it will be subjected. Current treatment methods also call for instrumentation which is able to provide both rigid fixation and semi-rigid or dynamized fixation that allows the implant to accommodate graft settling.
In anterior cervical applications in particular, orthopedic implants must be of very small size given the anatomical constraints related to work in this region. As a result, any implant to be placed in this region must be not only of diminutive length and width, but must also be of very small height in order to not protrude into the adjacent tissue or visceral structures. Ideally, it has been found that implants having a total thickness on the order of about 3.0 millimeters or less are acceptable. However, many prior art devices have total thicknesses in the range of 3 to 4 mm thick or more. Such thicknesses increase the risk of damage to the surrounding tissue, esophagus, and other vital anatomical structures.
An additional problem encountered with anterior cervical instrumentation is that, over time, the fixation screws tend to back out or loosen with respect to the remaining implant components. Such loosening, while sometimes benign, often leads to failure of the instrumentation systems. Screw loosening or screw migration can have catastrophic consequences in the case of cervical spine fixation, where a loose screw can penetrate the esophagus causing infection. Furthermore, it is important that bone screws are not installed using such a high torque as to strip the bone threads, especially when poor quality bone is encountered.
A number of systems employ various mechanical methods for preventing screw back out, nearly all of which employ some sort of secondary locking screw. Such a secondary locking screw increases the complexity and cost of the device, the surgical time required for implantation, and increases the chance for error on the part of the surgeon. Furthermore, other systems have attempted to employ a locking ring located in the shaft of the bone fixation screw. Such devices are inadequate in several respects. First, due to the anatomic constraints in the upper cervical region, screws employing a locking ring in their shaft necessarily are too long for proper use in cervical implantation. In order to have a sufficient length of threads and a sufficient length of shaft, the resulting screw is too long for safe use. Furthermore, screws employing a locking ring in the shaft usually employ a head section that must rest on top of the implant plate. Such arrangement is not desired, and indeed can be dangerous, as it results in an implant structure having a screwhead protruding above the plate. This protrusion can lead to infection as well as possible penetration of the surrounding tissue and anatomic structures.
As a result, the need exists for an implant system that provides a means of locking the bone fixation screw into the implant to prevent backout or screw migration. In addition, the need exists for such a system to be provided in implants suitable for small surgical areas, such as the upper cervical region. This necessarily requires a system that can be made of very small size, yet still have the desired locking properties. In addition, the need exists for such a system wherein the head of the screw is able to sit flush with or below the uppermost surface of the implant plate. Furthermore, the need exists for such a system that allows the surgeon to activate the locking mechanism using relatively minimal force, and in which the locking mechanism requires relatively great force to disengage.